The proposed investigations are in the field of regulation of arginine and urea synthesis in mammals and, in particular, in man. Urea cycle enzymes of normal human liver will be isolated and characterized as regards their structure, kinetics, and immunological and regulatory properties. Sensitive kinetic and radioimmunoassays will be developed to make possible the measurement of activity and specific protein of all of the urea cycle enzymes in small samples of tissue. These assays will facilitate the exact diagnosis of congenital deficiencies of the urea cycle enzymes, and possibly the prenatal diagnosis of some of these diseases. The long-term regulation of urea synthesis will be approached through studies on the adaptation of the urea cycle enzymes and other metabolically related enzymes of rat liver and kidney to different levels of protein intake. Work on the short-term regulation of this cycle will examine the effects of different protein and ammonia loads on the levels of urea cycle and other related intermediates in the liver. This information, together with that on the properties of the enzymes from human liver, will expand the basis on which to design the treatment of patients with congenital deficiencies of the urea cycle enzymes.